An ultrasound imaging apparatus which creates images of the three-dimensional internal structure of an object by using ultrasound waves is being widely utilized in medical practice as an ultrasound diagnostic apparatus that is inexpensive and with minimal side effects.
The performance of ultrasound imaging apparatuses is dramatically improving year by year based on the improvement of ultrasound imaging technology. As one technology to further improve the performance, research is being conducted on image reconstruction technology using the CMP (Constrained Minimization of Power) method (refer to Non Patent Literature 1). The CMP method referred to herein is also referred to as the DCMP method (Directionally Constrained Minimization of Power), or the CAPON method.
The CMP method is signal processing technology that was developed as one type of adaptive antenna technology, and is one type of adaptive signal processing. The CMP method is a reception method of adaptively adjusting the directionality of reception based on a constrained condition of evening out the received gain of the radio waves arriving from the intended direction, and constantly causing the power of all received signals, including interfering waves, to be minimum. According to this method, since the ratio of the interfering wave power can be minimized relative to the signal power, signals with a favorable SN ratio can be received.
The specific calculation of the CMP method can be basically executed with the respective steps of (1) to (6) below.
(Step 1): The ultrasound received signals received by a plurality (n number) of receiving elements are subject to delay processing, and the phases are matched so that the n number of ultrasound received signals generated from the same target position are aligned to the same time.
(Step 2): The n number of phase-matched received signals are converted into complex signals. For the ensuing explanation, the n number of complex signals at time t are set as a receiving complex vector X[t] configured from an n number of elements.
(Step 3): A complex covariance matrix A[k] is calculated with a given time period T clock based on the receiving complex vector X[t]. The calculation formula is Formula (1) below.
                    [                  Math          .                                          ⁢          1                ]                                                                      A          ⁡                      [            k            ]                          =                              ∑                          t              =              kT                                      kT              +              T              -              1                                ⁢                                          ⁢                                    X              ⁡                              [                t                ]                                      ⁢                                          X                ⁡                                  [                  t                  ]                                            H                                                          (        1        )            
Here, symbol H as the superscript of X[t] represents the complex conjugate as the vector transpose.
(Step 4): An optimal weight vector W[k] is calculated using the matrix A[k] and a known constrained vector C. The calculation formula is Formula (2) below.
                    [                  Math          .                                          ⁢          2                ]                                                                      W          ⁡                      [            k            ]                          =                                                            A                ⁡                                  [                  k                  ]                                                            -                1                                      ⁢            C                                              C              H                        ⁢                                          A                ⁡                                  [                  k                  ]                                                            -                1                                      ⁢            C                                              (        2        )            
Here, −1 as the superscript of A[k] represents the inverse matrix of A[k]. Moreover, the constrained vector C is a known vector for designating the arriving direction of the signals, and is a vector which normally causes all elements to be 1 relative to the output signal of the delay processing.
(Step 5): The constrained minimization of power Pow[k] is calculated from the optimal weight vector W[k] and the receiving complex vector X[t] based on Formula (3) below.
                    [                  Math          .                                          ⁢          3                ]                                                                      Pow          ⁡                      [            k            ]                          =                              1            2                    ·                                    ∑                              t                =                kT                                            kT                +                T                -                1                                      ⁢                                                  ⁢                                                                                                                      X                      ⁡                                              [                        t                        ]                                                              H                                    ⁢                                      W                    ⁡                                          [                      k                      ]                                                                                                  2                                                          (        3        )            
However, if Formula (3) is substituted with Formula (1) or Formula (2), it can be modified as Formula (4) below. Accordingly, the constrained minimization of power Pow can be directly calculated based on Formula (4) by omitting the calculation of the weight vector W[k].
                    [                  Math          .                                          ⁢          4                ]                                                                                                                Pow                ⁡                                  [                  k                  ]                                            =                            ⁢                                                1                  2                                ·                                                      ∑                                          t                      =                      kT                                                              kT                      +                      T                      -                      1                                                        ⁢                                                                          ⁢                                                                                                                                                                X                            ⁡                                                          [                              t                              ]                                                                                H                                                ⁢                                                  W                          ⁡                                                      [                            k                            ]                                                                                                                                      2                                                                                                                          =                            ⁢                                                1                  2                                ·                                                      ∑                                          t                      =                      kT                                                              kT                      +                      T                      -                      1                                                        ⁢                                                                          ⁢                                      (                                                                                            W                          ⁡                                                      [                            k                            ]                                                                          H                                            ⁢                                              X                        ⁡                                                  [                          t                          ]                                                                    ⁢                                                                        X                          ⁡                                                      [                            t                            ]                                                                          H                                            ⁢                                              W                        ⁡                                                  [                          k                          ]                                                                                      )                                                                                                                          =                            ⁢                                                                    1                    2                                    ·                                                            W                      ⁡                                              [                        k                        ]                                                              H                                                  ⁢                                  A                  ⁡                                      [                    k                    ]                                                  ⁢                                  W                  ⁡                                      [                    k                    ]                                                                                                                          =                            ⁢                              1                                                      2                    ·                                          C                      H                                                        ⁢                                                            A                      ⁡                                              [                        k                        ]                                                                                    -                      1                                                        ⁢                  C                                                                                        (        4        )            
(Step 6): The logarithm of the power Pow[k] is calculated, and the k-th pixel (target position) of the output line image is set as a gray value q. The calculation formula is Formula (5) below.
[Math. 5]q=Log [Pow[k]]  (5)
The processing of this logarithm conversion is not necessarily required, but is normally performed to facilitate the visualization of the output image.
Note that, in the actual calculation, spatial average processing on the matrix A and the small positive number addition processing on the diagonal elements are concurrently performed in addition to the foregoing steps. Nevertheless, details regarding such processing are not directly related to the present invention, and the explanation thereof is omitted.
As a result of performing the foregoing calculations, it is possible to perform image reconstruction based on the CMP method. It is known that, by using the CMP method, it is possible to obtain images with improved resolution and contrast in comparison to images that are reconstructed based on standard delay-and-sum processing.